The present invention relates to a fixer for thermally fixing a copy sheet or the like, and more particularly to an abnormal temperature detector adapted for detecting overheat caused by an abnormal state in the heat control of such a fixer.
For thermal fixing of a copy sheet onto which a toner image is transferred in a copying machine two thin plate-shaped heating elements of large surface area are disposed in parallel with each other. The copy sheet is passed through the space between the elements while heating the elements electrically. A similar method uses upper and lower bar-shaped heating elements in parallel with each other between which the copy sheet is passed.
In the fixer where such methods are used, it is desirable to maintain the temperature in the fixer within a range optimal for fixing the toner transferred onto the copy sheet. For this purpose, a temperature controller equipped with a temperature detector such as C-A thermocouple is provided directly on or in the vicinity of the heating element in the fixer so as to maintain the fixer temperature at the optimal point.
In such a fixer, when any deterioration or breakdown occurs in the temperature controller, or when the temperature detector attached directly to the heating element receives electric noise from any other component of the copying machine, or when the temperature detector welded directly to the heating element is separated partially or entirely from the surface of the heating element, then normal control is not attainable with respect to the electric power supplied to the heating element of the fixer. Consequently, power supply to the heating element is rendered excessive to cause a burning phenomenon of the copy sheet such as blackening or combustion in case the copy sheet is jammed in the fixer.
The object of the present invention is to provide an abnormal temperature detector which is capable of detecting abnormal heating in a fixer in its early stage so as to prevent the above-described problem.